This invention relates to a novel liquid crystalline compound expressed by the formula (I) ##STR2## wherein R.sup.1 and R.sup.2 each represent an alkyl group or an alkoxy group independently of each other; ring A and ring B each represent 1,4-cyclohexylene or 1,4-phenylene independently of each other; and X represents a hydrogen atom or a fluorine atom, and a liquid crystal composition containing at least one member of said compound of the formula (I) and further, its use. The compound of the formula (I) is a useful compound as a liquid crystal material containing a conjugated hexenediene inside its molecule. 2. Detailed Description of the Related Art
Liquid crystal display elements making use of characteristics such as optical anisotropy, dielectric anisotropy, etc. of liquid crystal substances have been broadly applied to watches, electronic calculators, etc. Liquid crystal phases include nematic phase, smectic phase and cholesteric phase, but nematic phase is practically most general. Display mode in this case includes those of TN (twist nematic) type, DS (dynamic scattering) type, guest-host type, DAP (deformation of aligned phases) type, etc. A number of liquid crystalline compounds used therefor have so far been developed, but any example wherein only a single compound has been filled in a display element and practically used, has not yet been found. As liquid crystal materials for display elements, it is necessary to exhibit liquid crystal phases in a broad temperature range around room temperature, be stable to moisture, light, air, etc. and to electric field and electromagnetic irradiation in the environment where they are used, and have physical properties sufficient to drive the display elements.
However, these conditions cannot be satisfied by only a single compound. Thus, several kinds of liquid crystal-line compounds have been mixed and further, non-liquid crystalline compounds have been mixed, whereby compositions according with such conditions have been prepared and practically used.
The values of physical properties such as those of optical anisotropy, dielectric anisotropy, electric conductivity, etc. sought for liquid crystal compositions depend upon display mode and shape of element. Among the values of the properties, the optical anisotropy value (.DELTA.n) of display materials requires a definite value, depending upon the cell thickness (d). For recent display elements, a method of reducing the value (d) to thereby obtain a high quality display having no domain, has been employed. Thus, in order to adjust the .DELTA.n value of liquid crystal compositions to an optimum value, liquid crystalline compounds having a large .DELTA.n value have become an important key to the above adjustment.
As compounds having a large .DELTA.n value, the following compounds have so far been generally known:
a tolan derivative (e.g. a tolan derivative disclosed in Japanese patent gazette laid-open No. Hei 01-502823; see chemical formula 11 mentioned below, or a butadiyne derivative (e.g. Mol. Cryst. Liq. Cryst. 48, 175 (1978); see chemical formula 12 mentioned below). But the former compound has a drawback of a narrow liquid crystal temperature range and the latter compound has a drawback of thermal unstability. Thus, both of the materials cannot be regarded as sufficient for achieving the above object.
Further, a stilbene derivative already known as a compound (e.g. a stilbene derivative disclosed in Flussige Kristallen. Leipzig. VEB Deutcher Verlag fur Grundstoff Industrie, 1975, 49 : see chemical Formula 13 mentioned below),
and a difluorostilbene recently developed in order to stabilize it (e.g. difluorostilbene derivative disclosed in Official Gazette of Japanese patent application laid-open No. Hei 03-294386: see chemical formula 14, mentioned below) are also useful as a liquid crystal material having a relatively large .DELTA.n and low viscosity, but either of the derivatives have no sufficiently broad liquid crystal temperature range; hence in order to overcome this defect, they could not have helped being used in admixture with other liquid crystal materials having a high clearing point. In order to develop novel compounds overcoming the drawbacks of these materials, the present inventors have completed a compound having an enyne structure conjugated within its inner part (an enyne derivative disclosed in Japanese patent application No. Hei 05-281147; see the following (chemical formula 15): ##STR3##
However, it is also difficult to regard the last-mentioned compound as exhibiting a sufficiently large .DELTA.n value. Thus, it has been long desired to develop a novel material having a very large .DELTA.n value, a broad liquid crystal temperature range, having a good compatibility with other liquid crystal materials, a low viscosity and a high reliability.